Studies in Saccharomyces cerevisiae have identified homologous recombination as the major mechanism for repairing DNA double-strand breaks induced by ionizing radiation. During the repair process, the ends of the DNA breaks are processed nucleolytically to yield 3' ssDNA tails, which are bound by recombination factors. The nucleoprotein complex thus formed then conducts a search to locate an undamaged DNA homolog and catalyzes the formation of heteroduplex DNA with the homolog. The RAD5O, RAD5I, RAD52, RAD55, RAD57, RAD59, MRE11, and XRS2 genes are key members of the evolutionarily conserved RAD52 epistasis group that mediate mitotic and meiotic recombination and the recombinational repair of DNA double-strand breaks. The RAD5O, MRE11, and XRS2 encoded products are associated in a complex that plays a central role in double-strand break end-processing. Rad5l protein, with the aid of ancillary factors, nucleates onto the ssDNA tails to form a nucleoprotein filament that has the ability to initiate heteroduplex DNA formation. A combination of biochemical and genetic approaches will be used to (i) delineate the functions of Rad5O, Mre11, Xrs2, and the complex consisting of these proteins, (ii) identify protein factors that functionally and physically interact with the Rad50-Mrel l-Xrs2 complex in DNA end- processing, and (iii) dissect the mechanism of action of Rad5 l, Rad52, Rad55, Rad57, and Rad59 proteins in heteroduplex DNA formation. The results from these studies will be important for understanding the mechanism of DNA double-strand break re air b recombination in eukaryotes including humans.